The present invention relates to an ink supply amount adjustment method and apparatus for a printing press, which adjust the amount of ink, to be supplied to a printing plate, by adjusting the ink fountain key opening ratio.
As shown in FIG. 12, an inking device (inker) in a printing unit of each color in a sheet-fed offset rotary printing press includes an ink fountain 2 which stores ink 1, an ink fountain roller 3, a plurality of ink fountain keys 4-1 to 4-n juxtaposed in the axial direction of the ink fountain roller 3, an ink ductor roller 5, and an ink roller group 6. Opening ratio of the ink fountain keys 4-1 to 4-n are adjustable. A printing plate 7 on which an image is printed is mounted on a plate cylinder 8.
In an inking device with such an arrangement, the ink 1 in the ink fountain 2 is supplied to the ink fountain roller 3 from the gap between the ink fountain keys 4-1 to 4-n and the ink fountain roller 3. The ink 1 supplied to the ink fountain roller 3 is further supplied to the printing plate 7 through the ink roller group 6 by the ink feed operation of the ink ductor roller 5. The ink 1 supplied to the printing plate 7 is printed on a printing sheet through a blanket cylinder (not shown).
A printing product 9 printed by the printing press has a band-shaped color bar 9-2 printed in a margin other than an image region 9-1, as shown in FIG. 13. In case of general four-color printing, the color bar 9-2 has a plurality of regions S1 to Sn including density measurement patches (solid patches with a percent dot area of 100%) 9a1, 9a2, 9a3, and 9a4 of black, cyan, magenta, and yellow. The regions S1 to Sn correspond to the key zones of the ink fountain keys 4-1 to 4-n in the printing unit of each color in the printing press.
[Color Matching]
A reference density value is preset for the printing unit of each color. That is, reference density values are individually set for respective colors, i.e., black, cyan, magenta, and yellow. In printing the printing product 9, a color matching operation in which the density value of each color is matched with the set reference density value takes place. The color matching operation is performed by an ink supply amount adjustment apparatus (not shown), before the start of final printing, based on the densities of the patches 9a1, 9a2, 9a3, and 9a4 of respective colors in the color bar 9-2 printed on the printing product 9.
The region S1 on the printing product 9, for example, will be described as a representative. The density value of the density measurement patch 9a of each color on the printing product 9 (test printing sample) obtained by test printing or proof printing before the start of final printing is measured. The density difference between the measured density value and the preset reference density value is obtained for each color. The correction amount of the opening ratio of the ink fountain key 4-1 in the printing unit of each color (the correction amount of the amount of ink to be supplied to the region S1) is obtained based on the obtained density difference for the color. The opening ratio of the ink fountain key 4-1 in the printing unit of each color is adjusted using the obtained correction amount for feedback.
Similarly, the correction amounts of the opening ratios of the ink fountain keys 4-2 to 4-n in the printing unit of each color (the correction amounts of the amounts of ink to be supplied to the regions S2 to Sn, respectively) are obtained for the regions S2 to Sn, respectively. The opening ratios of the ink fountain keys 4-2 to 4-n in the printing unit of each color are adjusted using the obtained correction amounts for feedback. After the opening ratios of the ink fountain keys 4-1 to 4-n are adjusted, test printing immediately restarts to repeat the same operation until the density value of each color reaches the reference density value (Japanese Patent Laid-Open No. 2003-118077: patent reference 1).
However, in the ink supply amount adjustment method disclosed in patent reference 1, the ink transfer path (the transfer path from the ink fountain roller to the blanket cylinder) in the inker is long. Therefore, to adjust the amount of ink to be supplied to a printing product, about 100 printing products must be printed from when the ink supply amount adjustment is complete until the ink supply amount adjustment result is reflected on an actual product so that the ink supply amount stabilizes at the corrected density. Therefore, this method requires much time and wastes printing materials.
In recent years, as disclosed in U.S. Pre-Grant Publication No. 2007/022888 (patent reference 2), the density value of the patch 9a of each color on the test printing sample 9 is measured to obtain the density difference between the measured density value and the reference density value of this color. The correction amounts of the opening ratios of the ink fountain keys 4-1 to 4-n in the printing unit of each color are obtained based on the obtained density difference for this color to correct the opening ratios of the ink fountain keys 4-1 to 4-n. 
After the opening ratios of the ink fountain keys 4-1 to 4-n are corrected, the ink feed operation of the ink ductor roller 5 is performed while printing is suspended, to adjust the amount of ink in the inker, and printing is then performed. A process of performing the ink feed operation of the ink ductor roller 5 while printing is suspended, after the opening ratios of the ink fountain keys 4-1 to 4-n are corrected, is called “preliminary ink feeding” in patent reference 2.
However, in the ink supply amount adjustment method disclosed in patent reference 2, preliminary ink feeding is performed upon correcting the opening ratios of the ink fountain keys 4-1 to 4-n. At this time, the correction amounts for use in preliminary ink feeding are obtained from only the density differences. Therefore, as for a portion with a low image area ratio (i.e., a portion with a low ink fountain key opening ratio) in a range corresponding to each of the ink fountain keys 4-1 to 4-n, if the density is so low that the ink supply amount is deficient, that portion is supplied with ink in an amount much larger than the required amount and has too a high density. Conversely, if the density is so high that the ink supply amount is excessive, that portion is supplied with ink in an amount much smaller than the required amount and has too low a density.
In contrast, as for a portion with a high image area ratio (i.e., a portion with a high ink fountain key opening ratio) in that range, if the density is so low that the ink supply amount is deficient, that portion is supplied with ink in an amount smaller than the required amount and remains to have a low density. Conversely, if the density is so high that the ink supply amount is excessive, that portion is supplied with ink in an amount larger than the required amount and remains to have a high density. This makes it impossible to print a normal printing product immediately after preliminary ink feeding. Therefore, a large number of printing products must be printed for adjustment until a normal product is obtained.